For over twenty years now it has been assumed that there is a black and white divide between bone histology and thermophysiology. Automatic endothermic “warm blooded” animals tend to show a haphazard composition of bone deposition, in which bone is laid down around surrounding blood vessels very quickly, with interspersals of more organized bone deposition (for strength). The term, coined by histologist Armand de Ricqles (1980), is fibrolamellar bone.
In contrast, bradymetabolic “cold-blooded” animals tend to show a regular deposition of layered, or lamellar zonal bone. This bone is not as well vascularized as fibrolamellar bone, and is often deposited at a much slower rate.
Back in 1980, this evidence was used along with a chain of other circumstantial evidence to show that dinosaurs were actually “warm-blooded” animals (Bakker, 1980). This challenge did not go unanswered, and even back then there were people questioning the evidence being proposed in favour of dinosaurian automatic endothermy. As far back as 1982, there were authours claiming to have histological evidence of fibrolamellar, “warm-blooded” bone growth in crocodylians (Ferguson et al, 1982). This evidence has often been scoffed at as being questionable at best (Horner & Padian, 2004). Skeptics have pointed out that the fibrolamellar crocodylians mentioned have all been captives. Being kept in a stable environment with easy access to food has resulted in these skewed results. Wild individuals would doubtfully show these traits, as access to scenarios like those provided in captivity, are unlikely.
For awhile this seemed to keep the argument of fibrolamellar bone, strictly in the pro-automatic endotherm camp. Well, not anymore.
Tumarkin-Deratzian, A.R. 2007. Fibrolamellar bone in adult Alligator mississippiensis. Journal of Herpetology. Vol. 41. No.2:341-345.
This paper reports the observation of long bone histology in alligators from Lake Griffin in Lake County, Florida. The findings are most interesting. Seven specimens were studied. Of these, three had extensive fibrolamellar growth in their long bones. In fact, one could put a fibrolamellar individual next to a lamellar zone individual and it would look like one was comparing a “classic mammal” to a “classic reptile.” The difference is incredibly dramatic; even moreso than comparing frame A with frame C in the above picture.
That’s not the best part though. You see, these lake Griffin alligators were not only wild animals, but they were stressed animals too. Currently the Lake Griffin alligator population is suffering from an intense die off. The reasons behind the high mortality at Lake Griffin remain uncertain, but there seems to be a link to thiamine deficiency in the animals dying.
This means that, not only are we seeing different bone deposition patterns in animals from the same population, but we are also seeing them from animals that were living under stressed conditions. This throws the whole “crocodylians can only show automatic endothermic growth rates under perfect conditions” argument right out the window.
So what does fibrolamellar deposition really show? Currently it remains unknown. It might still indicate faster growth. What it doesn’t indicate, though, is the thermophysiological preference of the animal in question.
Id est: it doesn’t seperate the “warm-bloods” from the “cold-bloods.”
More to come. Stay tuned.